I made a jig out of some 0.1 inch thick plexiglass with 0.060 inch shimming
around the edges. The shim is the same thickness as the circuit
boards and allows them to be sandwiched tightly with the vellum image.

The board is indexed into a corner and the vellum is taped
in position (face down) over the board. The top piece is held in
place with rubber bands.

This assembly is then placed about 4 inches
under one of those 'maginifier' florescent desk lamps and exposed for
about 30 minutes.

(I have switch to using a 75
watt halogen spot bulb on a jig at about 18 inches. This does
a nice exposure in about 4 minutes)

The newer type of presensitised boards (Ever-Muse) do not need ultraviolet
light. These boards, allong with the developer, are available from
Circuit Specialists

The dry mix for making one quart is only about a dollar. Since the
instructions claim that it must be used within 24 hours after being mixed,
I only mix up enough for each board. The usual, ferric cloride,
is then used for the etching. Don't forget to remove the etch-resist
coating from the traces before soldering components. The local $1
store has acetone (sold as nail polish remover) that work well for this.